Rename inappropriately named 'mask' fields to 'proposed_flags' instead.

Flags pending validation were stored in a misleadingly named field, 'mask'.
As 'mask' is already used to indicate pieces of a flags field which are
changing, it seems better to use a name reflecting the actual purpose of
this field. I chose 'proposed_flags' as they may not actually end up in
'flags', and in an case will be modified when they are moved over.

This affects the API, but not ABI of the user-mode interface.